Method of induction-refining a ferrous melt using a sponge iron charge

ABSTRACT

THE CRUCIBLE OF AN INDUCTION FURNACE IS SURROUNDED BY AN UPRIGHT COIL THAT IS ENERGIZED BY ALTERNATING-CURRENT ELECTRIC POWER. THE POWER INPUT TO THE COIL IS OF SUCH A LEVEL THAT A RAISED, SLAG-FREE CROWN IS FORMED ON THE TOP OF THE MELT AND AT LEAST 55% OF THE SLAG PRODUCED IS RETAINED TO FORM AN ANNULUS AROUND THIS CROWN. THE CENTRALLY ASCENDING AND PERIPHERALLY DESCENDING CURRENTS IN THE FURROUS MELT CAUSE THE CROWN TO FORM AND ALSO INDUCE THE ANNULUS TO INTROVERT (CIRCULATE INWARDLY AND DOWNWARDLY) SO THAT LIGHT SPONGE IRON ADDED TO THE MELT IS DRAWN DOWNWARDLY INTO THE SLAG-STEEL INTERFACE AND QUICKLY HEATED AND BROUGHT INTO INTIMATE CONTACT WITH BOTH FLOATING SLAG AND THE FERROUS MELT. THE POWER IS FED TO THE COIL IN A MAGNITUDE DETERMINED BY THE RELATIONSHIP P&gt;K(F**(1/2)) WHEREIN P IS THE POWER INPUT IN KILOWATTS PER TON OF THE MEL,T, K IS THE ANNULUS-FORMING CONSTANT AND IS GREATER THAN 45 AND PREFERABLY MORE THAN 49.5, AND F IS THE FREQUENCY IN HZ. OF THE ALTERNATING CURRENT.

1974 HEINZ'DIETER PANTKE ETAL 7 METKOD OF INDUCTIQNREFINING A FERROUSIELT USING A SPONGE IRON CHARGE Filed Oct. 2, 1972 SLAG FOFMERS K PovvzeSUPPLY Patented Aug. 6, 1974 Patent Ofice ME'II IQDnOFINDUC'HON-REEINING A FERROUS MELT USING A SPONGE IRON CHARGEqherhausenosterfeld, Germany," assignors to Hutten- Contin'afion-in-par't of application'Ser. No. 228,406, Feb.

17? 22, 1972, -whichi's' a continuation of application Ser.

879,459,- Nov=."" "26; -1969,'hoth now abandoned. This ":iapplication00152, 1-972,Ser. No. 294,436

'Claims ,tpriority;application.Germany, Nov. 29, 1968,

US. CI. 75-12 4 Claims '=ABSTRACT OF THE DISCLOSURE crucible ofaninduction furnace is surrounded by a'niupright coil that is energized:by alternating-current electric power. The power input to the coil is ofsuch a level that a raised, slag-free crown is formed on the top ofthemeltanilat least 55% of the slag produced is retainedtoforma i annulusaround this crown. The centrally ascendingand:peripherally descendingcurrents in the" ferrous melt cause the crown to form and also inducethe annulus to iritrover'tj(circulate inwardly and downwardly) so thatlight sponge iron added to the melt is drawn downwardly into theslag-steel interface and quick- 1y "heated and brought into intimatecontact with both floating slag and: the ferrous melt. The power is fedto thecoil ina magnitude determined by the relationship wherein} is thepower input in kilowatts per ton of the melt, K is the annulus-formingconstant and is greater than 45 and preferablymor'e than 49.5, and f isthe frequency in Hz. of the alternating current.

1; Cross Reference to Related Applications i lflhe"p'rjesent;application' is" a cohtinuation-in-part of application No. 228,406 filedFeb. 22, 1972 (now abandoned) as a continuation of application Ser. No.72,l59 filed Nov. 2-6, 1 969 andnow abandoned.

am i. 2.-Field ofthe Invention Thei' preserrt invention "relates to amethod of incorporating light-weight additives "in a ferrous melt and,,nioreparticularly, to a steel-makin'g'refining process which mploys acrucible-type induction furnace and a spongeiron'cha'r'ge. i g IBackground of the Invention 1 A' crucible type" induction furnacegenerally comprises "a crucible su'rrouiided by an electric coil whichis ener- 'gized by 'high-ener g'y alternating current. Anelectromagnetic field is thus produced which generates eddy currents inthe 'c'harge in the crucibleto heat and melt the charge. :In the. caseof aferrous melt, the field has a radially in- -war'dlyacting componentwhich displaces the charge so that a stirring and convective current isdeveloped which rises at the 'center of the melt and'descends at thesides thereof. --'A= -fu'rnace' of; this type has been proposed for the'addition 'of'substa'nces-"to 'a 'steel melt at a slag-free crown, themajor part" and preferably all *of the slag being-removed from the"charge."

Such a -furnace' has never been successfully applied,

'h-owever; to -the large-scale smeltingand refining of steel "fromhigh-purity sponge iron a'sisproduced, for example, by the -direct gas'reduction 7 of iron ore. Such furnaces have been proposed for thesmelting and refining of scrap orthe heating'of liquid nfetals fromother sources; Lightig-weight. low-=densitynor .low z-specificgravitymetals. and

nonmetallic or nonconductive materials, e.g. slag formers, sponge iron,light alloying metals and compounds and other refining additives andreactants, are not sufficiently heated because of high porosity, lack ofintimate contact with the liquid phase and the low density, and merelylie on the slag topping the melt in the absence of a crown or upon thesurface of the latter in spite of the dynamic or circulating movementthereof. It is, for example, virtually impossible to melt sponge iron ina conventional induction furnace.

A solution to the above problem has been the additional use of suchmeans as a plasma burner above the melt to heat and melt any lighterelements added from above and floating on the melt. This answer to theproblem, however, requires the installation of additional apparatus withadded cost and problems.

4. Objects of the Invention It is, therefore, an object of the presentinvention to provide an improved method of treating ferrous melts,especially steel.

More specifically, it is an object of the present inven tion to providesuch a method which makes possible the large-scale refining andproduction of steel with an induction furnace overcoming theabove-described disadvantages and enabling the use of sponge iron as thecharge to be refined.

Another object of the instant invention is to provide an improved methodof adding solid materials to a ferrous-metal melt which will bring thesolid phase and the liquid phase into intimate contact at a high rate,thereby enabling rapid heating of the lightweight and normally floatingmaterials as well as early reaction between the phase at theirinterface.

It is a further object of the invention to provide an improved method ofincorporating steel-refining additives in a ferrous melt upon which alayer of slag or other relatively lightweight material is, or has beenformed.

It is still another object of the invention to provide an improvedmethod of melting sponge iron which eliminates the need for plasmaburners and auxiliary heating means and which enables sponge iron tocontact all parts of the charge of a steel-making installation withoutdisadvantages generally accompanying the low-density and highheat-dissipation rate of sponge iron.

5. Summary of the Invention stirring of the melt.

An essential feature of this invention resides in a method ofincorporating solid materials into a ferrous melt, this methodcomprising the steps of electrodynamically stirring a ferrous melt (e.g.the melt in a steelrefining furnace), forming an.-introve'rting annulusof slag at the top of the melt around a'slag-free crown so that theflanks of the crown descend'outwardly while the inner portion of theslag annulus curls inwardly in 'a vortex-like action, and depositing thesolid materials on the slag-free'crown' so that they are entrainedbetween theflanks of" the crownand the 'int'roverting slag, therebypassing into intimate surface contact with boththe slag :and the furnacemelt. The specific power (he; power per unit weight of charge) must besufiicient to raise the crown while at least 55% of the slag producedmust be retained during refining, i.e. at least 55% of the slag presentand generated from each furnace-operating cycle must be retained duringthe next operating cycle.

When the solid phase is sponge iron, i.e. the highly porous,lightweight, high-purity product of the direct gas reduction of ironore, the normal tendency of the sponge iron to float upon the surface ofthe melt is countered by the forcible entrainment of the slag betweenthe outwardly descending flanks of the crown and the inwardly anddownwardly curling flanks of the slag annulus to sandwich the spongeiron between slag and ferrous melt.

As a consequence, the iron is drawn rapidly downwardly and into the bodyof the melt so that it cannot dissipate the heat transferred to it andit is rapidly smelted or dissolved in the ferrous portion of the liquidface.

Furthermore, the intimate contact of the freshly added iron with theslag of the annulus andany slag which may be entrained into the body ofthe ferrous phase, results in a substantially instantaneous reactionbetween the slag and the freshly smelted iron as the latter is added orshortly thereafter. Thus, practically all of the newly added, unrefinediron (essentially sponge iron) is refined into steel without delay and amore or less continuous process may be practiced.

In accordance with this aspect of the invention, therefore, refinedsteel may be tapped from the ferrous phase of the crucible of theinduction furnace while a corresponding quantity of sponge iron iscontinuously added at the raised crown of the charge in the furnace,preferably together with additional slag-forming materials andadditives. Instead of continuous addition and withdrawal of charge andrefined metal, an intermittent discharge and charge process may beemployed.

In this connection it has been found to be important to retain at alltimes 50-60% of the melt capacity of the crucible in the latter, whilethe balance of the metal charge may be tapped 01f, thereby retaining abody in which the electrodynamic stirring can be sustained in the mannerpreviously described. Only the remainder of the melt is drawn off aftera preceding refining cycle and the crucible contains 50-60% by weight ofits original capacity of the molten metal and any residual slag in theform of an introverting annulus.

As previously described, sponge iron and any slag formers necessary aremetered gradually onto the slagfree crown with compensating increase inthe induction power to maintain the stirring and heating.

It is thus a feature of the present invention that the power input tothe induction furnace is of such a level that a slag-free crown isformed on the upper surface of the melt and rises well above the edgesof the melt so that the slag forms its inverting or introverting annulusaround this crown. Since the high power makes for a rapidly moving,centrally rising and peripherally descending cur-rent in the melt, thisannulus introverts continuously inwardly, in contact with the descendingflanks of the crown. The introversion causes any additive to the melt,cast upon the slag-free crown, to be drawn into the interface betweenthe essentially nonconductive slag and the ferrous melt where it isbrought into an excellent initimate contact with the molten steel someportions of the slag actually being entrained by the current into thebody of the melt. In this manner, any lighter-thansteel materials can bereadily melted or dissolved by the molten steel since they will notremain atop the slag where they otherwise might fail to pick upsufficient heat. In the same manner, the molten steel continuously con-.tacts the slag across a conitnuously changing interface so that thedephosphorizing or desulphurizing additives can easily accomplish theirfunctions.

According to an essential feature of the present invention, thealternating-current electric power is applied the induction coil of thefurnace at such a level following critical relationship applies:

that the wherein P is the applied electric power in kilowattsper ton ofmelt, f is the frequency of the-power in Hz, and

preferably 400 kw./ton is used;

According to another feature offthejinyention', jtli additives arecharged into lt he crucible atop the s'la'gffree crown and thencedrawh'iritiofjth'e melt-slagf'interf ace Moreover, it is found-thatfo'nly 'a'part, of.t he m elt, e.'g. 40 to 50%, should be drawnotfas rnol'tensteel to be replaced with sponge iron in equivalent amount; the slagbeing discharged-whenever it tends 'to, coveruthe crown of the ferousmelt. Means is provided to regulate the power input to the furnaceindependence upon the dimensions of the charge therein.

As noted, the slag is removed whenever it forms such a thick layer thatit is likelysto, vcover the entire top of the melt, including the top ofthe raised crown. In this covered conditionadditi-ves such asslag'forrn'ers, iron ore, or sponge iron cannot be charged directly ontothe molten steel. H i v We have found that it is essential for thepurposes of the present invention that, during the refining process,there should be retained in the crucible at least 55% of the slagproduced from the refining of a full metal charge therein. Thus, at thestart of'a subsequent refining operation, there is retained in thecrucible at least'55 by weight or volume of the slag originally presentand formed in the previous operation 'of the' rama e; Additional slagwill then form in the subsequent operation and may be continuously drawnoff or retained or off in increments as long as the slag layer does'notcoir'ipletely cover the crown. We have also found that there is acritical basicity value of the Slag which should be maintained toprovide the desired viscosity and A coreless induction furnace withcapacity of 1000 kg. of steel, as described at page 793 of MarksMechanical Engineers Handbook, MeGraw-Hill Book Company, 1958,isJ'char'ged to capacity with steel scrap to which limestone af'ndsilicafslag formers .have'been added. I

After initial smelti'ng the charge is brought'to a temperature abovethe' melting point and heated at a final power of 400 kw. (400, kw/ton)with 50 alternating current. 55% of the 'charge, ,and alljof the slag,are retained in the furnace and about 45% of the steel is tapped off.The power is reduced to 70% of its' original level (i.e. 280 kg.corresponding to 508 kvmper tort of charge). Sponge iron, as made asdescribed in the commonly assigned applications, pending-with the parentapplication thereof, Ser. No..71 1,102, Ser., No. 839,45l and Ser. No.834,065 (filed6 March 1968, 7 July 1969 and 17 June 1969, now U.S.Pats..3,5ll,390, 3,598,257, and 3,591,158, respectively). and theearlier applications mentioned therein, serves as the steel making.charge. with the usual slag former and is, introduced intothe partiallyempty crucible in which electrodynamic stirring is maintained at theindicated power. level; the. granulated sponge iron is graduallydropped. onto. the slagfree crown and .is drawn between the slag,annulus and the descending flanks of the crown. The power. isincreased Igradually to its original level of; 400 k-w., (40Q- kw./t on) and assoon as the charge reaches 100 of capacity as noted above, the refinedsteel is partially tapped and the procedure repeated with a minium of55% of the slag from a previous run being retained for the next run.

7. Description of the Drawing The above and other objects, features, andadvantages will become more fully apparent, reference being made to thesole figure of the accompanying drawing, in which an induction furnaceaccording to the present invention is diagrammatically illustrated,largely in vertical section.

8. Specific Description The furnace comprises an upright crucible 2 ofgenerally cylindrical shape surrounded by a similarly cylindricalcoil 1. The crucible is nonconducting electrically and is preferablycomposed of a refractory material. Above the crucible is a hopper 9which can adjustably charge the crucible 2 with slag formers and otheradditives, and with iron ore, sponge iron, and the like. Anoxygen-ejecting lane 10 can, if necessary, be employed to decarburizethe melt.

The hoppers 9 have their outputs metered or regulated by a controlarrangement 12 which may also be sensitive to the melt temperature. Thisdevice 12 is connected to a power supply 13 for the coil to maintain theabove-described power ratio. As the size of the melt is increased byaddition of material from the hoppers 9, the control arrangement 12increases the power input to the coil 1.

The entire crucible 2 and coil 1 can be sealed off and evacuated at 14if desired. Molten steel is tapped through pipe 15 and the entirecrucible can be tilted to run oif slag or a separate tap may be providedabove the surface of the melt for this purpose.

Thus, in a normal running condition of the furnace according to theinvention, a body of molten steel 3 has liquid thermal-convective andelect-romagnetically induced currents 5 forming an axially elevatedcrown 4 on its upper surface when and P 350 kw./ ton, for example. Thus,an annular body 6 of slag having introverting currents is formedsurrounding this crown. Material 8 is added as necessary by the hoppers9 to the uncovered raised crown.

In a first stage of steel refinement, the readily oxidizable componentsof the melt are burned ofi as they react with components of the slag,the freshly added material, and with the atmosphere, since the centralcrown 4 is exposed to the atmosphere. At the same time, a reactionbetween the iron (II) oxide and the phosphorus in the melt, for whichreaction the melt must be relatively cool, basic, and have a highoxidation potential, removes this phosphorus from the steel.Subsequently, the sulphur must be removed. For desulphurization the slagmust be of a reducing character, relatively low in iron (II) oxide, andof a temperature (both of slag and steel) which is as high as possible.

After initially forming the melt from scrap steel, for example, slagformation commences and smelting of new metal occurs at the bathtemperature slightly above the melting point. To obtain eifectivedephosphorization the melt carbon content is set to a maximum of 0.1% byweight, preferably 0.03% to 0.07% by weight. This can be accomplished bythe addition of ore to the melt, directing a jet of oxygen against themelt, or by other known means. The reaction thus produced lowers thephosphorus content of the melt. Then the refining slag is run off to theextent it exceeds, taking into consideration later-formed quantities ofslag, the minimum of 55% for retention previously set forth.

Subsequently the melt is recarburized by simultaneously heating it andsupplying the necessary additives. Basic oxides are also added todesulphurize the melt as it is being heated and recarburized. Once thesteel attains its intended composition or the desired state ofequilibrium, the crucible can be capped and evacuated to stop thereaction. 7

Advantageously, only 40% to 50% of the steel is drawn off, leaving allthe original slag and S0 to 60% of the steel forthe next batch, at thislevel, the power at the coil is 70 to 78% of capacity as adjusted tocompensate for the reduced charge.

As mentioned above, it is important to adjust the quantity of slag sothat the steel itself is bared at the raised crown. In addition, theaforestated minimum quantity of slag must be retained if the vitalslag/metal interface and the osculation between introverting slag andextroverting metal are to trap and hold the charge for the necessarytime. In this manner sponge iron can be charged onto the uncovered crownin powdered, granulated, or lump form and is drawn by the currents 7 and5 into the interface between the melt and the slag where it iscompletely surrounded by very hot material and rapidly melted. It isimportant that the power input to the heating coil be increased as newmaterial is added, allowing for the normal smelting exothermicreactions. Slag formers (limestone and siliceous materials) are addedalong with the sponge iron. Obviously, once the layer of slag developsto the point that it covers the raised crown, a portion of it is runoff.

According to another variation of the method, one wherein a minimalamount of slag is produced, only about half of the intended quantity ofsponge iron is added to the melt, without any dephosphorizing slagformers. Whatever slag is formed is run off, then the rest of the spongeiron is added along with basic oxides which form slag, the manganesecontent being so chosen that the slag formed thereby just suflices totake up enough phosphorus to set the proper phosphorus content in thefinished steel. This slag may be retained for the next charge.

Of course, alloying materials can be added just prior to tapping toproduce whatever exact composition in the steel is desired.

9. Comparative Example The following comparative example is intended todemonstrate the criticality of the aforementioned power limitation inrendering the system of the present invention capable of processing suchlightweight materials as sponge iron by contrast to relatively denseparticles or bodiesas have been processed heretofore in inductionfurnaces. It will be recalled that the improvement constituting thepresent invention derives from the use of a certain minimum energy sothat a crown of sufficient height is created and the use of a minimumslag level to ensure the proper duration of contact at the interfaceinto which the sponge iron is drawn. The slag layer should also be of acertain viscosity to ensure that it will remain an introverting layer,this being controlled in part by regulating its basicity.

More specifically, the invention is designed to refine (desulfurizeand/or dephosphorize and/or decarbonize) an iron melt into which acharge of sponge iron is introduced. The sponge iron charge has aparticle size of l to 30 mm., a porosity of 50 to 60%, a bulk density(including pores) of about 3 g./crn. and contains small quantities ofsilicon dioxide, calcium oxide, magnesium oxide, aluminum oxide andtitanium oxide in addition to a predominancy of iron and the usualproportions of phosphorus, carbon and sulfur. Slag has a specificgravity of 3.5 to 4.5 g./cm. and liquid steel a specific gravity of 7g/cm. The heat conductivity of the iron sponge is only one sixth to onefifth that of steel. In the absence of a substantial slay layer,particles of sponge iron as thus described do not obtain sufficientcontact, when deposited on the surface of the melt, to becomeassimilated into the latter in an efiicient manner. The presence of asubstantial slag proportion is therefore essential.

. I n a,2 -ton induction. crucible furnace with a normal operating/powerof. 800 kw. at a frequency of 50 Hz. (corresponding to'400 ,kw./ t.),sponge iron made from the direct gas reduction of Itabiraore pellets isrefined to reduce thesulfur, phosphorus and carbon levels.

Thechemical composition of the sponge iron is:

Iron by weight. 92.5 Silicon dioxide do 2.08 Calcium oxide do 1.65Magnesium oxide -do- 0.18 Aluminum and titanium oxides do 1.04 Sulfur do0.003 Phosphorus do 0.031 Carbon do 0.70 Density (including pores)g./cm. 2.9 Particle size mm 1 to 15 The basicity B of the slay wasadjusted to 1.4 by the addition of burned lime, the latter consisting ofthe slagging medium. The basicity is defined as Percent Ca Percent MggPercent SiO Percent A1 0 It has been found that a basicity of this orderof mag nitude, e.g. ranging upwardly of 1.2, will provide the desiredviscosity and, at the same time, enable the phosphorus level in the meltto be reduced significantly. Tests 1 and 2 below demonstrate this point:

Test 1 Test 2 Established electric power, kW/t 350 300 Formation ofintroverting crown Good Temperature of the molten bath, "0 1. 480- 1,530- Electric energy consumption per hour, kWh 645 390 Attainedconnection time, percent 75 65 Raw steel quantity in melt, kgJh 1,030530 Slag quantity, kg./t. raw steel 85 110 Yield of metallic product,percent 99 93 Specific power consumption, kWh/t. raw steeL 625 740Analysis, percent:

When the process is carried out without an initial slag layer or whenthe slag is continuously tapped to remove a' predominant proportion, thepower consumption per ton 4 (a) continuously refining an iron melt inthe presence of slag formers to produce molten steel and a layer of slagthereon in an electric induction crucible furnace;

(b) continuously tapping 50 to of the molten steel of step (a) from saidfurnace While retaining therein at least 55% Of the slag produced instep (a);

(c) continuously inductively heating and electrodynamically stirring themelt remaining in said furnace following step (b) with analternating-current magnetic field generated by an alternating currentapplied to said furnace at a power P per ton of the melt, where- K545and f is the frequency of the alternating current in Hz., to raise anoutwardly moving central crown of bare metal on said melt withdescending flanks of molten steel inducing said layer of slag into theconfiguration of a continuously introverting annulus surrounding saidcrown;

(d) continuously depositing on said raised crown sponge iron in aparticle size of 1 to 30 mm. in an amount substantially equivalent tothe amount of steel tapped from said melt in step (a) and entraining thesponge iron into the interface between the molten steel and introvertingannulus;

(e) refining the sponge iron incorporated into said melt in step (d);

(f) withdrawing slag formed in steps (a) to (e); and

(g) charging additives to the steel in said furnace onto said crownsubsequent to step (f).

2. The method defined in claim 1 wherein K is 549.5.

3. The method defined in claim 1 wherein said power is at least 350kw./t. of the melt.

4. The method defined in claim 3 wherein said power is at least 400 kw./t. of steel of the melt.

References Cited UNITED STATES PATENTS 3,579,324 5/1971 Kennedy 1O1,940,622 12/1933 Clamer 13-27 1,838,527 12/1931 Clamer 1327 1,946,8732/1934 Neuhouss 7512 L. DEWAYNE RUTLEDGE, Primary Examiner P. D.ROSENBERG, Assistant Examiner U.S. Cl. X.R. 13-27; 7511

